A row-crop harvester such as described in German patent publication No. 1,582,211 has a frame displaceable along the ground and forming a plurality of forwardly open throats each adapted to receive a respective row of crop during operation of the harvester. An endless conveyor chain is provided at each of these throats spanned over a drive sprocket and an idler sprocket, with a conveyor stretch of the chain extending along one edge of the respective throat and another return stretch of the chain being provided adjacent this conveyor stretch. The drive for the harvester is coupled to all of the drive sprockets to advance the chains and thereby draw the stalks of row crop backwardly along the throat to the chopper of the harvester.
In the known devices the idler sprocket is urged away from the drive sprocket by means of a spring arrangement to keep the conveyor chain under tension. In case the conveyor chain breaks this tensioning arrangement is set up so as to pinch the broken conveyor chain between the idler sprocket and an abutment to prevent the conveyor chain from entering the chopper of the harvester. This conveyor chain is subject to considerable wear so that its breaking is not a rare occurence. Similarly if the conveyor chain enters the chopper of the harvester considerable damage will be done, so that it is essential to prevent the chain from working free in the event of breakage.
The known systems have several disadvantages. First of all the biasing arrangement typically becomes less effective as the chain stretches with age. This is due to the fact that normal springs are less stiff as they decompress. Furthermore if the chain breaks at or very close to the idler sprocket it is possible for it to work free and enter the chopper of the machine. Another disadvantage is that it is impossible for the operator of the machine to ascertain when a chain is near the breaking point or has so badly stretched as to be likely to break.